Rowan,
In truth
NO, but I do recall some mention of dropping the gear if it malfunctioned being mentioned before, may have been on the series "Wings" on TV. I'm still looking, but again the memory fails the Horror - the Horror.
The only reference I could find that actually mentions jettison, is for the single wheel modification added for the SC500 bomb Feisler designed an explosive bolt jettison for that, to be added to 109s specifically to give ground clearance on take off.
EDIT - Upon further reading, Doh, it appears that the lever in the cockpit is as Merlin said "to release the gear to lower by gravity incase of engine failure".
Sorry.